This invention relates to an exhaust-gas turbocharger for the two-stage supercharging of an internal-combustion engine with a construction to prevent losses of lubricant from a bearing housing for the bearings of the compressor and the high-pressure exhaust-gas turbine.
In the case of two-stage supercharging of internal-combustion engines by exhaust-gas turbochargers the bearing of the shaft of the high-pressure stage is frequently arranged in a bearing housing between the turbine wheel and the compressor wheel, while this housing serves as a catching tank for the oil leaving the bearings, out of which it flows back, or is conveyed by a pump, into the oil sump of the engine or into an oil tank separate from the latter. Because it is impossible to seal the above-mentioned housing relative to the compressor housing and the turbine housing so that the entry of oil into them is prevented, the pressure in the bearing housing must always be lower than the pressures in the compressor housing and turbine housing respectively. However, the pressure differential must not be so great as to result in sacrifices of efficiency in compressor and turbine. The pressure in the bearing housing should therefore be only as little as possible below the respectively lower of the pressures in the compressor housing and turbine housing.
As mentioned, the oil which flows out of the bearings into the bearing housing flows from there either into the oil sump of the engine or into an oil tank separate from the latter, while provision must be made for breathing in both cases. The above-mentioned desideratum of the smallest possible pressure differential between the interior of the bearing housing, in which the ambient pressure prevails, and the compressor housing and turbine housing is also dictated by the fact that, in the case of too great a pressure differential, too much air passes into the relevant oil collecting tank and inadmissibly large quantities of oil mist are therefore produced which escape through the oil tank breather and/or crankcase breather into the atmosphere and contribute to environmental pollution.
There is the further consideration that, for small pressure differentials, it is possible to use unsupported ring seals for the sealing means between the bearing housing and the compressor housing and turbine housing; these have a better sealing effect than supported ring seals and are also cheaper.
In the case of single-stage turbochargers, in order to cool the turbine side shaft stub and in order to prevent the entry of exhaust gases into the bearing housing and into the compressor, blocking air is branched out of the boost air pipe and passed through blocking air ducts to the transitional region from the turbine side shaft end to the turbine wheel and escapes into the atmosphere. With this design the labyrinth seal between the compressor housing and the compressor wheel is constructed with such a radius that the axial thrusts acting upon the compressor wheel and the turbine wheel are substantially mutually cancelling. The air behind the labyrinth is likewise discharged into the atmosphere.
Attempts are made by producers of turbochargers to make it possible for such turbocharger types originally designed as single-stage to be used as the high-pressure stage of two-stage turbochargers with the fewest possible modifications. The most important changes and adaptations to the higher pressures in front of the compressor and behind the turbine which are most important for this purpose consist in measures which raise the pressure in the blocking air ducts and in the bearing housing so high that the small pressure differential referred to initially is obtained.
A device is known from the German Utility Design GM No. 80 26 375 of Messrs. Dr. Ing. h.c. F. Porsche AG, whereby losses of lubricant resulting from the causes initially described are intended to be prevented in single-stage turbochargers for spark-ignition engines. For this purpose the air space of an oil tank, in which the oil discharged from the bearing housing is collected and which serves an an air separator is connected to a point of the intake pipe located between a throttle valve and the compressor. Depending upon the position of the throttle valve, which serves as a power regulating element, a higher or lower negative pressure prevails in the admission pipe in front of the compressor, which is transmitted into the oil tank and into the bearing housing, so the emergence of oil from the latter into the compressor housing is prevented, and simultaneously the oil mist formed in the oil tank passes back via the admission pipe and the compressor into the combustion chambers of the engine. Because, due to the negative pressure mentioned, the oil does not flow back spontaneously into the crankcase of the engine, it is necessary to provide a pump which conveys the oil back into the crankcase.
As state, this device is intended for Otto engines which exhibit a throttle flap in the admission pipe in front of the compressor. Single-stage or two-stage turbochargers of diesel engines do not exhibit a throttle flap in the admission pipe, and are therefore not suitable for the system described.